1. Field of the Invention
The present invention relates to a slip control apparatus for a four wheel drive vehicle and, more particularly, to a slip control apparatus for a four wheel drive vehicle in which a slip on each of the wheels on pavement can be controlled.
2. Description of Related Art
When wheels of vehicles, as automobiles, slip on pavement in a large degree of slippage during running, they cannot grip the pavement so that appropriate running characteristics cannot be achieved. Accordingly, in order to control a slip of the driven wheels on pavement, a slip control apparatuses are known which can operate a brake unit installed in the vehicle or which can reduce output of the engine.
In controlling the slip of the driven wheels on pavement by such slip control apparatuses, it is required to detect a degree of slippage of the driven wheel on pavement greater than a given value or to detect a vehicle velocity in order to set a target circumferential velocity or a slip ratio of the driven wheels slipping on pavement in a degree of slippage greater than a given value. In a two wheel drive vehicle in which only the front wheels or the rear wheels are driven, it is relatively easy to detect a vehicle velocity on the basis of a circumferential velocity, namely, a rotational velocity, of undriven wheels which are generally less frequent to slip on pavement. However, a four wheel drive vehicle is another story because its four wheels are all driven and there is no such undriven wheel.
In order to overcome the difficulties with which such a four wheel drive vehicle encounters, Japanese Patent Publication (Kokai) No. 289,429/1987 proposes a slip control apparatus for a four wheel drive vehicle, in which a vehicle velocity is estimated on the basis of the least circumferential acceleration out of the circumferential acceleration of the four wheels while a slip of each of the wheels in an excessive degree of slippage on pavement is detected on the basis of the circumferential acceleration.
In such a conventional slip control apparatus for the four wheel drive vehicle, however, the engine output is controlled to be reduced, on top of control over operation of a brake unit, only when all the driven wheels slip in an excessive degree of slippage on pavement because this adversely affects a running stability of the vehicle to a great extent. When three driven wheels or less are slipping in such an excessive degree of slippage on pavement, an application of a braking force by the brake unit to such slipping wheels is controlled in order not to reduce driving characteristics such as acceleration, running performance on rough road and so on, while no control is made for reducing the engine output which serves as reducing the driving force of the vehicle to a remarkable extent.
It is considered, however, that even when three wheels or less of the four wheel drive vehicle are slipping in such an excessive degree of slippage on pavement, the output of an engine is controlled to be reduced, in addition to control over operation of a braking unit, in order to ensure a stable running performance in accordance with running states of the vehicle. In this case, there is the opportunity that driving characteristics be reduced to an extreme extent by having the driving torque acting upon the wheels reduced to an extent more than necessary.
In particular, when two of the wheels slip in an amount greater than a given value, it should be noted that a degree of adverse reactions upon the running stability of the vehicle varies to a great extent with a combination of two wheels slipping out of six combinations in which the two of the four wheels are combined with each other. Thus it is desired that a slip control be done while the combination of the two wheels slipping to a degree greater than the given value is taken into consideration, in order to ensure the running stability of the vehicle as well as to control a reduction in the driving characteristics of the vehicle. For a four wheel drive vehicle, for instance, when the front and rear wheels on the left-hand side or on the right-hand side are slipping in an amount of slippage above the given value on pavement, an application of the breaking force by the brake unit to such slipping wheels may make a difference between the driving torque acting upon the slipping wheels and the driving torque acting upon the corresponding non-slipping wheels too large, thereby incurring the risk of causing a yawning that is not desired for the running stability. As another example, when the left-hand and right-hand front wheels in the four wheel drive vehicle are slipping in such an excessive amount, an application of the braking force by the brake unit to those front wheels increases torque to be transmitted to the left-hand and right-hand rear wheels so that it causes the tendency the vehicle is oversteered. In this instance, if the engine output would be reduced, the driving torque to all the wheels would be caused to be reduced controlling a decrease in the running stability of the vehicle. Conventional four wheel drive vehicles, however, are provided with no slip control apparatus which enables a slip control by taking into account a combination out of six combinations of two wheels which are slipping in an amount greater than the given value.